1. Field of the Invention
The present invention relates to a method for manufacturing a multi-domain liquid crystal cell, and more particularly, to a method for manufacturing a multi-domain liquid crystal cell in which a liquid crystal cell of a multi-domain liquid crystal cell having a wider viewing angle is obtained by a simple process.
2. Discussion of the Related Art
Recently, a liquid crystal display (LCD) mainly used for portable televisions or notebook computers requires a large sized screen, so as to be used for a wall type television or a monitor. A twisted-nematic (TN) liquid crystal cell is generally used as an LCD. The TN liquid crystal cell has different optical transmittivity characteristics at each gray level depending on viewing angles. For this reason, a large area of the TN liquid crystal cell is limited. That is, optical transmittivity is substantially symmetrical in view of a viewing angle in left and right direction while optical transmittivity is asymmetrical in view of up and down direction. Accordingly, an image inversion area exists in the viewing angle in up and down direction. As a result, there is a problem that the viewing angle becomes narrow. To solve such a problem, there is suggested a multi-domain liquid crystal cell in which a compensation effect of a viewing angle is obtained by varying a main viewing angle in each pixel. To obtain the multi-domain liquid crystal cell, a reverse rubbing process will be described with reference to FIG. 1.
As shown in FIG. 1a, an entire substrate lion which a polyimide 12 is deposited is processed by rubbing. Thus, a mono-domain is formed as shown in FIG. 1b. As shown in FIG. 1c, one domain is blocked by a photoresist 13. Rubbing is then performed in a direction opposite to the rubbing direction of FIG. 1a. As shown in FIG. 1d, a domain which is not blocked by the photoresist 13 is processed by reverse rubbing. As shown in FIG. 1e, if the photoresist 13 is removed, a substrate divided into two domains having opposite pretilt angles can be obtained.
However, the liquid crystal cell manufactured by the above rubbing process has problems in that dust or static electricity occurs during the rubbing process, thereby reducing yield or damaging the liquid crystal cell.
In another related art, to solve such problems, photo-alignment methods based on UV rays have been suggested instead of rubbing One photo-alignment method will be described with reference to FIG. 2. As shown in FIG. 2a, a substrate 21 on which an alignment film 22 is deposited is periodically shielded by a mask 23 having a light-transmitting portion 25 and a light-shielding portion 24. When light (solid line arrow in tilt direction on a top of the drawing) is irradiated at a tilt at an angle of xcex8, a first pretilt is determined in a portion 26 where light is transmitted. As shown in FIG. 2b, the mask 23 is rearranged to shield light in the portion 26. Then, when light (solid line arrow in tilt direction on a top of the drawing) is irradiated at a tilt at an angle of 31 xcex8, a second pretilt is determined in a portion 27 where light is shielded in FIG. 2a. Thus, as shown in FIG. 2c, a first substrate of two domains having different pretilts can be obtained. Also, as shown in FIG. 2d, a second substrate of two domains can be obtained by the above alignment method and lower and upper substrates are assembled with each other.
Furthermore, as shown in FIG. 3, areas Q and R are light-shielded, and the photo-alignment methods having an angle xcex8 of irradiation in FIGS. 2a and 2b are sequentially applied to areas O and P as shown in FIGS. 3a and 3b. The areas O and P are then light-shielded, and the photo-alignment methods having an angle xcex8 of irradiation in FIGS. 2a and 2b are sequentially applied to the areas Q and R as shown in FIGS. 3c and 3d. Thus, light irradiation of total four times is performed on the substrate, thereby obtaining a substrate having four domains.
As described above, the first substrate and the second substrate in which four domains are formed are bonded to face each other and then the liquid crystal is injected thereto, so that a four-domain liquid crystal cell can be obtained.
In still another related art, as shown in FIG. 4a, a semi-transparent portion 43 of a mask is arranged in some area of a substrate 41 on which an alignment film 42 is deposited, and then light irradiation is performed. Thus, the irradiated light is absorbed in the alignment film 42 on the substrate 41 in an aperture portion. However, some of the irradiated light is only absorbed in the alignment film 42 in an area of the alignment film 42 of the substrate 41 corresponding to the semi-transparent portion 43 of the mask. Polysiloxane based materials used as the alignment film 42 are characterized in that the size of the pretilt angle becomes small as absorbing light energy increases. Accordingly, the size of the pretilt angle formed in the alignment film can easily be controlled. Based on this characteristic, a substrate having different pretilt angles and divided pixels is manufactured, and a sectional view of the substrate is shown in FIG. 4b. As shown in FIG. 4c, a liquid crystal cell is manufactured in such a manner that upper and lower substrates are bonded to each other by applying the substrate of FIG. 4b. In this structure, alignment direction of each domain is identical in each substrate but the size of the pretilt angle is different. Accordingly, a multi-domain is formed to improve a viewing angle. Also, in case that the divided pixels are applied, a four domain liquid crystal cell can be obtained as shown in FIG. 5. In this case, areas III and IV are light-shielded and the photo-alignment method of FIG. 4a is applied to areas I and II (see FIGS. 5a to 5d). Subsequently, the areas I and II are light-shielded, and in the areas III and IV, a four-domain substrate is obtained by varying polarization direction of the irradiated light in the photo-alignment method of FIG. 4a (see FIGS. 5e to 5h). After the first and second substrates in which four domains are formed are bonded to each other by the above method, the liquid crystal is injected into the substrates so as to obtain a four-domain liquid crystal cell.
However, the methods for manufacturing a liquid crystal cell through the photo-alignment methods have several problems in controlling alignment direction of the multi-domain to realize a wider viewing angle.
The first method requires light irradiation of eight times (vertical irradiation of four times and tilt irradiation of four times) into the upper and lower substrates to form a multi-domain divided into two pixel areas, and mask bonding process of four times. Moreover, to form a multi-domain having four domains, the process steps increase two times. Thus, in addition to light irradiation and mask bonding processes of several times, a gap between the masks and the substrate should additionally be controlled. These process steps are unattractive in view of the mass production. The second method requires light irradiation of four times (vertical irradiation of two times and tilt irradiation of two times) into the upper and lower substrates and mask bonding process of two times when forming a multi-domain having two domains. In the second method, the light irradiation and the mask bonding process steps have been reduced but error may occur in arranging a number of masks, thereby reducing the productivity.
Accordingly, the present invention is directed to a method for manufacturing a multi-domain liquid crystal cell that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method for manufacturing a multi-domain liquid crystal cell in which a photo mask required for photo-alignment is improved to perform tilt irradiation having two different directions by irradiation of one time, so that alignment division of a unit pixel can be realized and a multi-domain liquid crystal cell can be obtained by a simple process.
Another object of the present invention is to provide a method for manufacturing a multi-domain liquid crystal cell in which the number of masks is reduced to reduce error that may occur in arranging the masks due to control of a gap between the masks and the substrate.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the scheme particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for manufacturing a multi-domain liquid crystal display device having a pixel comprising the steps of: forming an alignment film on at least one of first and second substrates; covering the alignment film with a mask, the mask including a first surface having a plurality of light-transmitting portions and light-shielding portions and a second surface having light-shielding portions corresponding to the light-transmitting portions of the second surface; irradiating light from an upper portion of the mask; and assembling the first and second substrates.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.